Those cops definitely dropped the ball. In an over an hour interview they totally missed any red flags.
The cops probably within five minutes knew they were dealing with psychologically disturbed lowlife and got a pretty good idea about the troubled relationship between the pretty girl with insecurities and demons and the bad boy lowlife. But they are not therapists and without a formal complaint or evidence , they cannot make an arrest, howver in retrospect they and the parents of those two wish they had. BTW without a body, there will be no conviction or justice. Wasn’t long ago in America that the male relatives of this girl would meet with her boyfriend and in twenty minutes get the detailed story. Sorry but very much doubt this girl has any truly masculine relatives these days.